Modified Futterman OTL amplifier

82.55.220.250

Thanks to tubetvr and Lew suggestions I have just built and tested this OTL amplifier. The schematic is different from my previous candidate scheme (see message dated April 8th, 2008). This is a modified Futterman design (in order to get the lowest output impedance possible)with some moderate feedback. The tubes are (4X)6080/(1X)6SN7/(1X)12AT7 per channel. To me it sounds very good.I think this amplifier is not too difficult for DIY. I am open to deliver the complete schematic (power supply and amplifier) to all interested readers (e-mail a request to cellai@space.it).

I don't know how I might have helped, but it sure looks cool. One reason it may sound better than some other Futterman designs is simply due to the use of the 6SN7, presumably at the input. I tend to think the 6SN7 outshines most 9-pin miniature dual triodes or the pentodes that are used in the classic Futterman circuit.

The original files I had submitted for posting are fully adequate for reading. The problem apparently lies in the way the jpg files are treated by this site. I will be glad to send the original files to the e-mail address of your choice (some other readers have already done this way). Please send an e-mail with the address to cellai@space.it.

RE: Schematics (1 of 2), posted on June 14, 2008 at 02:54:11

Koonw

Do you have different or same value of plate and cathode resistor in phase splitter (can't read clearly in your schematic)? Because it can make a lot differences as this elimatated a lot extra global NFB that degraded the sonic, especially the upper treble and also speed. As your driver is quite simialar to old rca otl amp. http://www.miduho.com/jp/st/amp-kairo/rca/OTL%20Amplifier/rcaotl.gif it has 56K and 33K in phase splitter. Near very other otl amps use Gobal NFB in one way or another or NFB in the driver stage though, can you test is it better?

The old 'rca otl amplifier' is very similar to the 1954 'Dickie & Makowsky scheme' I had used for the otl guitar amplifier that I have posted here some months ago.

It has about 40 dB of negative feedback (as a matter of fact the final stage made of 12AT7, 6SN7 and 3X-6080 has unitary gain because the output signal returns directly to the cathode).

I have noticed that so much negative feedback results in weird effects in case of hard clipping.

This is the main reason why I have tried to trade in more gain (from the very same tubes) in exchange for a lower negative feedback.

In my proposed scheme (I will be glad to send better jpeg files at the e-mail address of your choice) the open loop gain (@ 8 Ohm)is about 116 (23*1.8*14*6.5mA/V*4*8) whereas the closed loop gain is about 16.

To me this give the best sonic results with my speakers (ESB 7/08, four way Italian floor speakers manufactured in 1989).

Coming to your question the plate resistor of the first triode from the left is 27K (with a current of 2.2 mA) and its cathode resistor (i.e. NFB resistor) is 10K.

You should see here the input phase splitter of the 'modified Futterman schematic'.

There is also a second phase splitter (see the second triode from the left, 33K plate and cathode resistors, 2.5 mA) but its function (in tandem with the following 6SN7 tube differential amplifier) is only to boost the differential drive of the totem pole (4X) 6080 tubes (originating from the 27K plate resistor) to up to 135 V p-p.

The reason why many OTL schematics with 6SN7 differential drive and 6080 tubes exhibit a 18K plate resistor lies in this basic fact: there is no way to have 135 v P-P in the load with a different plate resistor.

Can OTL amp really good guitar amp? I am too interested. The clipping distortion and recovery is not the same as transformer output, but may be I can do something about it if you email the schematic to my email address. I think there are ways to make the clipping better 1)configure the amp in voltage follower instead of cathode follow mode, in the rca otl I believe it is in already in voltage follow mode which is prefered(?) 2)use positve feedback to ensure sufficient gain even at clipping and also can have better damping factor with lesses negative global feedback. 3) allow driver to clipp first before the output by shunting across the 2 plates of driver (2nd tube 6SN7 with 18K plate resistor) with a variable resistor say 250K-B or suitable choke. For driver level it's 160V P-P in upper arm and 120V P-P in lower arm. The gives 40V P-P or 25W into 8 ohms or 80V P-P or 50W into 16 Ohms. Like flaw in old Willaimson amp, this old otl amp may have same flaw, the cathode bias resitor my be too low a value which I hope it may be verified.

A professional reader has kindly simulated (SPICE model)the harmonic distortion of the output stage of the proposed OTL scheme WITHOUT FEEDBACK (open loop) and the result is:Fundamental (1 KHz) - 0 dB2nd harmonic - -25 dB3rd harmonic - -30 dB4th harmonic - -45 dB5th harmonic - -65 dBWith FEEDBACK applied (as in the schematic) the calculated improvement should be of the order of 17 dB (that is 2nd harmonic -42 dB, 3rd harmonic -47 dB and so on).As the professional reader explained to me (in a private mail) the open loop presence of even harmonics depends on non ideal push-pull operation. Potential DIY audiophiles may wish to improve further the design and to cancel even harmonics (in open loop, before feedback application) by reducing the imbalance of the drive signal of the output stage (for a deeper discussion on the topic and on noise reduction techniques see http://www.tubecad.com/articles_2001/totem-pole_output_stage_psrr/Totem-Pole_Output_Stage.pdf, particularly the last page).

In general Spice simulation of distortion is difficult and with 6C33C it is even more difficult. The main problem is to find a good enough model that resembles the real characteristics of the tube, the 6C33C exhibit some unusual behavior with a kink in the Ia characteristics at low currents and this is difficult to simulate in a model. I have used all available models for spice simulation of the 6C33C that I have found and no one give results that are accurate for distortion, the best I have found so far is from here http://www.excem.fr/download/usergui5.pdf

It is difficult to see from the schematics that you have posted but as far as I can see you have not included any balance compensation in the driver stage. As I wrote earlier a SEPP is unbalanced and need a special driver arrangement for compensation, the most well known is the Futterman. In the Futterman with split load phase inverter the cathode resistor is returned to the output rather than ground and this will compensate for the unbalance, but I can' see that you are using that circuit, (sorry but the resolution of the scematics is not good enough to see clearly).

In my own OTL where I use the Futterman connection the 2nd order product are 55dB down before feedback, I haven't removed the Futtterman connection and measured but it would certainly be much worse.

I am wondering if it is possible to partially compensate for the intrinsic gain imbalance of the output stage (with no feedback and without resorting to a Futterman phase splitter returned to output), at least in the case of a load equal to the nominal speaker impedance (8 Ohm).

The 6080 (double triode) tubes that I have used have a plate resistance of 300 Ohm and mu=2 (gm = 6.5 mA/V). The parallel of four 6080 tubes exhibits a nominal plate resistance (rp) of 300/4=75 Ohm.

Based on these values and standard formulas the nominal gain of the upper triodes is 0.1616 = mu/((mu+1)+rp/8 Ohm) and the nominal gain of the lower triodes is 0.1927 = mu*8 Ohm/(rp+8 Ohm).In other words the gain imbalance is 19.2% (0.1927/0.1616).

If one fixed 6 Ohm resistor were added in series with the cathodes of the lower triodes the corresponding gain would go down from 0.1927 to 0.1584 = mu*8 Ohm/(rp+8 Ohm+(1+mu)*6 Ohm) and the resulting nominal imbalance would be less than 3%.

Could it be an alternative (for even harmonic reduction) worth to try?

RE: Balance Compensation, posted on June 30, 2008 at 08:53:29

tubetvr

Yes, it would be possible to compensate for the gain difference but the output impedance from each tube would still be different meaning that the balance would be lost if the load impedance would vary as it does in a real speaker. The futterman connection, (and some other methods) compensate for the gain and the output impedance difference and the balance is therefore maintaned also when the load impedance vary.

I have feeling that you don't like the Futterman compensation, do you want to explain why?, (I am just curious).

There has been many papers written about methods for how to compensate the unbalance of the SEPP amplifier I can recommend the paper by Hiroya â€œAnalyses of Drivers for Single-Ended Push-Pull Stage,â€ Hiroshi Amemiya, IRE Transactions-Audio, Sept-Oct 1955" that is also mentioned by John Broskie

In the present context I would just prefer to compensate for the intrinsic imbalance of the output stage (I remind that the OTL amplifier is in use since a month ago and that I am quite satisfied of its current performance) by adding a simple resistor (rather than by adding a new 450-500 V rail and by introducing some strong modification of the current OTL schematic).

An Italian proverb (a sort of Murphy's law) states that when something is almost good enough, you should not modify it, otherwise it fails.

The following table shows the calculated variation of the gain of the upper and lower triodes of the output stage as a function of the speaker impedance (from 1 Ohm to 17 Ohm, assuming a fixed compensation resistor of 5.5 Ohm, that is, four 22 Ohm resistors in parallel).

The imbalance with no compensating resistor at all is 19.8%.

With a 5.5 Ohm compensating resistor (that is best for 8 Ohm speakers) the imbalance is less than one half (i.e. approximately 10%) when the speaker impedance varies from 4 to 14 Ohm. A good result for a simple resistor, don't you agree?

Last but not least I have also found that the sensitivity of the % of residual imbalance does not vary significantly with mu and rp: this is really intriguing.

RE: Balance Compensation, posted on June 30, 2008 at 12:23:45

tubetvr

Good result but maybe not good enough, 10% imbalance is more or less the same as 10% 2nd order distortion or -20dB, in a push pull stage I would look for at least -40dB or 1% which then calls for less than 1% unbalance for all loads.

As I described earlier my own amp has -55dB with 8 ohm and about -50dB with 4 ohm but then I also tune the phase splitter by adjusting one of the resistors, as I don't use matched output tubes it is necessary, without tuning the phase splitter I easily get -40dB or 1% 2nd order distortion.

Regards Hans

Experiment with compensating balance resistor, posted on July 9, 2008 at 06:43:01

I have provisionally tried a 5.4 Ohm (in series with the cathodes of the SEPP lower triodes) for fixed compensation of the SEPP nominal gain imbalance (0% nominal gain imbalance @8 Ohm).

This simple modification was mainly aimed at assessing if I was able to listen to some audible difference (with and without such a fixed balance compensation).

The limited listening test performed so far has been clearly based on impressions 'prior and after' the modification (hence not by a live comparison of the switched outputs of two simultaneously available amplifiers), as the OTL amplifier HW had to be always the same.

Theoretically the calculated 2nd harmonic (closed loop) distortion should have gone down from a supposed level of something less than -30 dB (no compensating resistor) to something better than -40 dB (with compensating resistor).

I must say that if there is a difference (and I believe there must be) it can be only ascertained by some direct measurement and spectral analisys (not available at this time).

By my ears only (over some hours of listening to my favourite classic music) I can neither say if there is a significant difference nor if the sound is subjectively better (sorry, probably I am not a 'golden ear').

This is the reason why I am not going to modify further the present OTL design.

A fully automatic SEPP gain imbalance compensation will be the matter of my next OTL (likely a true 'modified Futterman' like yours).

For sake of completeness the calculated compensating resistor for 16 Ohm speakers is 10.65 Ohm.

A spreadsheet is available to potentially interested DIY readers for calculation of compensating resistor (send a request to cellai@space.it).

This is the measured output spectrum with a low level 1 KHz tone applied at the OTL input. The OTL output level is the maximum possible without saturating the A/D converter of my PC audio card.

For measurements I have used the free SW (Visual Analyzer) available at: http://www.sillanumsoft.org/Visual Analyser is a complete professional real time (FREE) software that transform your PC in a complete set of instruments (no new hardware necessary, you can use the Sound Card of your PC).

The corresponding harmonic distortion of the OTL amplifier is lower than 0.1%.

In order to reduce the even harmonics (due to SEPP gain imbalance) a 5.4 Ohm fixed compensating resistor has been added to the schematic (see my previous mail on the subject).

The measured signal has been picked up in parallel to my speakers and I can assure that carrying out the measurement with a strong 1 KHz signal in your ears is not pleasant at all. I suppose this is the reason why dummy load resistors are used instead of real speakers.

In my opinion this measurement demonstrates that it is possible to obtain a reasonably good performance (with the proposed schematic) without resorting to the more complex 'true modified Futterman' that requires additional tubes and a 450 V additional voltage rail.

For sake of completeness I have also measured the output spectrum with no signal applied (see my next post).

Measured Noise Spectrum of the Modified Futterman OTL amplifier, posted on July 18, 2008 at 08:41:11

This is the (noise) output spectrum of the OTL amplifier with no input signal. Unfortunately the octave plot (that shows the peak centered on 100 Hz, that is, twice the frequency of European power lines) can not be captured by the current SW.

In this area I think there is room for improvement (by some additional filtering).

As a matter of fact it is completely hand made. Most components come from radio surplus stock (in particular the eight Sylvania 6080 NOS tubes) and the container is hand made from a dismissed VHS recorder. For DIY people I believe this is a plus.If you wish to receive some pictures of the interior please send me an e-mail address for delivery (by the way, what do you mean by 'ccty'?).

Hi Folks: Sorry to clutter the thread, but I too would like a clear copy of this schematic. A couple of the email addresses for sources on this thread seem blocked or no longer working. Thanks in advance. WS

I don't know how I inspired you in any way, but you do beautiful work. Thanks for the post, and I will be sending for the schematic, just to satisfy my curiosity.

I am wondering how to apply a minimal amount of NFB around the output stage of a circlotron, just to reduce output Z down to a level acceptable for driving my Sound Lab speakers. Goal is to get the output Z down to 1 or 2 ohms from 6 or 7 ohms.